1. Field of the Invention
This invention relates to a circuit arrangement for continuously adjusting the base width in a stereo decoder in which a sum signal and a difference signal of two reproduction signals are obtained from a received signal and are converted in a matrix by sum and difference formation to form the reproduction signals, and more particularly to such a circuit arrangement in which attenuation is controlled in the difference signal channel depending on the strength of the received signal.
2. Description of the Prior Art
The frequency-modulated stereo-multiplex signal conventionally employed for the transmission of two discrete information channels which, in stereo radio, contain the information from two recording devices for left and right, and accordingly for two reproduction loud speakers on the left and the right, is composed of three components. The first component is a sum signal (L+R) consisting of the left-hand information L and the right-hand information R in the frequency range from 30 Hz to 15 kHz. The second component is a difference signal (L-R) consisting of the left-hand information L and the right-hand information R which is modulated onto a suppressed 38 kHz auxiliary carrier. The frequency band for the difference signal (L-R) extends, from the lower to the upper side band, from 23 kHz to 53 kHz, the frequencies and ranges given herein being exemplary of FM stereo multiplex transmission. A third component serves to transmit a pilot tone of 19 kHz which permits the regeneration of the 38 kHz auxiliary carrier in the stereo decoder. This 38 kHz auxiliary carrier is connected to the pilot tone in a phase-locked fashion. In the stereo decoder, the sum signal (L+R) and the difference signal (L-R) in the original frequency state, i.e. from 30 Hz to 15 kHz are shaped, via the matrix, to form the reproduction signals U.sub.L and U.sub.R which are then fed to the corresponding reproduction devices. The conversion of the difference signal (L-R) from the 38 kHz state into the original frequency state is effected with a synchronous demodulator which is controlled by the regenerated 38 kHz auxiliary carrier. A stereo decoder of this type is compatible with both mono-and stereo-transmissions.
In the case of a mono-transmission, which contains only a sum signal (L+R), the difference signal channel is blocked. The same occurs when, in the case of a stereo-transmission, the receiving field strength is too low for the synchronous demodulator to operate satisfactorily. In these situations only the sum signal channel is transmissive; for this reason, the two reproduction devices both receive the same information. If, on the other hand, the synchronous demodulator supplies a difference signal (L-R) of full signal strength, then, in the ideal situation, both reproduction devices will receive completely separate signals. Then no cross talk occurs from the one channel to the others. In the case of complete channel separation, two corresponding reproduction loud speakers appear, to the person listening thereto, as original sound sources, the sensed distance between which is referred to as base width. The less the channels are separated from one another, i.e. the more one channel produces crosstalk to the other, and the more a loud speaker contains information from the other, then the more the sound sources appear closer together and the base width is smaller. In the extreme case of a mono-transmission, and in the case of a total crosstalk when both loud speakers are reproducing the same information, the base width has reduced to zero; the sound source appears to the listener to lie in the middle between the two actual loud speakers.
If during the reception of a stereo signal, the stereo decoder is switched over from mono to stereo, and vice versa, then, in terms of the reproduction, this appears as a transfer of base width from the value of zero to the maximum value and vice versa. In the case of fluctuating receiving field strengths, in particular in automobile radio receivers, this becomes manifest as a disturbing, constant, hard switch-over between mono and stereo.